1. Field of the Invention
The present invention relates to an optical fiber coupler used for optical communication, especially for fields of a short-distance transmission system and a fiber laser. The invention also relates to a multi-mode optical coupler with an integrated section of which configuration is controlled with parameters so as to optimize coupling efficiency of multi-mode light.
Priority is claimed on Japanese Patent Application No. 2007-299600, filed Nov. 19, 2007, the content of which is incorporated herein by reference.
2. Background Art
Optical fiber couplers (referred herein as “coupler”) are classified roughly into two groups as to their structures: couplers manufactured by heating two fibers arranged in parallel to fuse and integrate together so that light from one of the fibers is coupled laterally to the other (such pumping is called “side pumping”); and bundle couplers in which multiple fibers are bundled together so that light is coupled through ends of the fibers (such pumping is called “end surface pumping”).
These couplers are manufactured by, for example, heating multiple fibers to fuse and integrate together and then stretching the fibers to obtain couplers with a diameter-reduced integrated section. Couplers manufactured in this process are called fused fiber couplers.
The fused fiber couplers have often been used for optical communication, and are usually combined with single-mode fibers. Various couplers, such as a TAP coupler, a 3 dB coupler and a WDM coupler, are in practical use.
The coupler according to an aspect of the invention is a multi-mode optical coupler for coupling multi-mode fibers rather than single-mode fibers.
In general, multi-mode optical coupling is often used for supplying pumping light to a rare earth doped fiber in an optical amplifier (including laser). The multi-mode optical coupler according to an aspect of the invention is also suitably used for pumping light coupling in an optical amplifier.
Couplers used for coupling multi-mode light have been proposed in, for example, Japanese Patent No. 2980248, Published Japanese Translation No. 2000-502193 of the PCT International Publication and Japanese Unexamined Patent Application, First Publication No. 2005-241712.
Among these couplers, one disclosed in Japanese Patent No. 2980248 is simple in structure, easy to manufacture and stable to heat and vibration. In the structure of the coupler disclosed in Japanese Patent No. 2980248, however, coupling efficiency to a multi-mode fiber is determined depending substantially on a cross-sectional ratio of each fiber. For example, in a combination of one multi-mode fiber and one single-mode fiber which used as multi-mode fiber disclosed in Japanese Patent No. 2980248, coupling efficiency to the multi-mode fiber is 1/(1+1)=50%. In a combination of two multi-mode fibers and one single-mode fiber, coupling efficiency to the multi-mode fibers may be as low as about 1/(2+1)=33%.
In order to address such low coupling efficiency, a core of the single-mode fiber may be doped with a rare earth element for temporarily absorbing pumping light. In this case, the light coupled to the single-mode fiber is absorbed by the core and is thus lost. As a result, a larger amount of light carried in a multi-mode fiber becomes coupled to the cladding of single-mode fiber, and thus the coupling efficiency eventually exceeds 50%.
However, light is absorbed by the core typically slowly, and thus in order to improve coupling efficiency in the above-described manner, the portion where the fibers are twisted and integrated together becomes significantly elongated.
In the disclosure of Japanese Patent No. 2980248, it is not clear whether the light is coupled to the core or the clad of the single-mode fiber. But coupling to the clad would seem to be a natural interpretation and an integrated length required therefor will be estimated below. Under the circumstance described above, the single-mode fiber in Japanese Patent No. 2980248 may be considered as a double-clad fiber.
The light absorptivity of the core itself is assumed to be 500 dB/m. Since the light is spread out into the entire fiber, the amount of light effectually absorbed by the core becomes smaller depending on the cross-sectional ratio of core with respect to the entire fiber. The core diameter is usually about 10 μm and an outer diameter of the fiber is usually not less than 100 μm, which means that the cross-sectional ratio is up to 0.01. In this case, the effectual absorptivity is up to 5 dB/m. Accordingly, the integrated fiber length required for absorbing 90% of light in the single-mode fiber is at least 2 m. The coupling efficiency herein with the combination of one multi-mode fiber and one single-mode fiber is 75%.
As described above, if the light carried in the multi-mode fiber is to be coupled to a single-mode fiber with the coupling efficiency of not less than 50% in the process disclosed in Japanese Patent No. 2980248, a length of several meters will be required for the coupling. It is thus difficult to integrate the fibers and is thus not useful.
Disclosed in Published Japanese Translation No. 2000-502193 of the PCT International Publication and Japanese Unexamined Patent Application, First Publication No. 2005-241712 is another method for coupling light between a light carried in a fiber and a light laterally injected the fiber using space coupling. Such a method requires a complicated structure and has varying coupling efficiency depending on misalignment. Accordingly, there has been a problem that reliability may vary significantly due to environmental conditions, such as a temperature change, vibration and impact.